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Convection

Convection. Contributions by: Janet deGrazia, Stephanie Bryant Department of Chemical and Biological Engineering University of Colorado Boulder, CO 80309-0424.

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Convection

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  1. Convection Contributions by: Janet deGrazia, Stephanie Bryant Department of Chemical and Biological EngineeringUniversity of ColoradoBoulder, CO 80309-0424

  2. A hot plate is being cooled by flowing air. The heat transfer coefficient for turbulent air flow is______________ that for laminar air flow. less than greater than the same as Turbulent Flow Laminar Flow

  3. For cold liquid metal flowing over a hot flat plate in laminar flow, the thermal boundary layer will be ____________ the hydrodynamic boundary layer. larger than less than the same as

  4. For cold liquid metal flowing over a hot flat plate in laminar flow, the onset of fully-developed flow will take ____________ that of water. longer than shorter than the same as

  5. Hot water flows over a cold isothermal flat plate. If the water temperature increases, the heat transfer coefficient _________________ and the transition flow length ____________. The water is incompressible. increases, increases decreases, decreases stays the same, stays the same increases, decreases decreases, increases

  6. A hot cylinder is cooled by turbulent cross flowing air. Which graph could represent the convective heat transfer coefficient (h) as a function of the angular position around the cylinder (θ)? B. C. D. h 0 180 h h h θ 0 0 180 180 0 0 180 180 θ θ θ θ

  7. Flowing cold air cools an aligned 4 x 3 hot tube bank. The air is laminar before entering the bank. The convective heat transfer coefficient in row 3 will be _____________ that in row 1. greater than less than the same as 1 2 3 Cross flow air

  8. Laminar cross flow air enters an aligned tube bank. Converting the tube bank from an aligned arrangement into a staggered arrangement would ______________ the heat transfer rate. increase decrease not change Cross flow air Aligned Staggered

  9. Turbulent cold air flows over an aligned 3 x 3 hot tube bank. Doubling the number of tubes in the _________ direction, while maintaining the same tube spacing, would have the hottest outlet air temperature. longitudinal transverse either Longitudinal Transverse Cross flow fluid

  10. Laminar cold air flows over two aligned hot tube banks each with 8 tubes. One bank has 4 rows of 2 tubes each. The other has 2 rows of 4 tubes each. Which tube bank will have the greater outlet air temperature? B. Cross flow fluid C. D. • Not enough information • Both will have the same outlet temperature Cross flow fluid

  11. For incompressible flow within a cylinder, which statements are true? I. The mean velocity is best used to determine the Reynold’s number. II. When flow is fully developed, the velocity profile does not change with distance along the tube. III. Transition to turbulent flow has a lower Reynold’s number than for external flow. IV. Flow becomes fully developed faster for fluids with a high Prandtl number. A. I, II B. I, III C. I, II, IV D. I, II, III E. All the statements are true

  12. Water flows through a tube of length L with an mean inlet temperature (Ti). The tube is held at a uniform surface temperature (Ts) greater than Ti. Which temperature profile best represents the mean temperature of the water as if flows through the tube? C. D. Ts T T T Water T L Ti Ts Tube Ti

  13. Water enters a tube of length L with an mean inlet temperature (Ti). The water is heated with a uniform heat flux through the tube. Which temperature profile best represents the mean temperature of the water as if flows through the tube? C. D. Uniform heat flux T T T Water T L Ti Tube Ti Ti Ti Ti

  14. Water flows through a tube of length L with an mean inlet temperature (Ti). The tube is held at a uniform surface temperature (Ts) greater than Ti. Assuming fully developed, laminar flow, the outlet temperature (To) _______ when the mass flow rate is doubled. stays the same increases decreases Ts Water L Ti To

  15. Water flows through a tube of length L with an mean inlet temperature (Ti). The tube is held at a uniform surface temperature (Ts) greater than Ti. Assuming fully developed, laminar flow, the heat flow _______ when the mass flow rate is doubled. stays the same increases decreases Ts Water Ti L

  16. A spacecraft contains a freshly hard boiled egg. The atmosphere inside the spacecraft is air at room temperature and atmospheric pressure. While the spacecraft is in space, the rate of cooling is ___________ that while the spacecraft is on earth. greater than less than equal to

  17. A suspended spherical block of dry ice at -79°C is exposed to atmospheric air at 30°C. The air close to the dry ice moves _________. up down circular around the sphere nowhere Atmospheric air 30 °C Dry Ice -79 ° C

  18. A thin square plate is in a room held at an ambient temperature of 25 °C. One side of the plate is insulated while the other is kept at a constant 100 °C. Orienting the plate _________ will result in the most free convection heat transfer. vertically horizontally either way Vertical Horizontal Insulated

  19. B. C. D. E. A and B will have the same heat transfer rate A thin plate has one side insulated and the other is kept at a constant temperature. The ambient temperature is less than the temperature of the plate. Which orientation would result in the highest heat transfer rate?

  20. An array of fins and cold air are cooling an electronic chip. If the spacing of the fins is barely widened, while keeping the same individual fin geometry, the rate of heat transfer will _________________. increase decrease stay the same Not enough information vs.

  21. An array of fins and cold air are cooling an electronic chip. If the number of the fins is halved, while keeping the same individual fin geometry and spacing, the rate of heat transfer will _________________. increase decrease stay the same Not enough information vs.

  22. Which fluid would be better at removing heat from an array of fins? Assume all the fluids to be at ambient temperature and are quiescent. Air Ethylene glycol Hydrogen

  23. Laminar air flows over an isothermal plate. The local heat transfer rate _________ as the position in the x direction increases, because the thermal boundary layer ___________. decreases, decreases increases, decreases decreases, increases increases, increases stays the same, stays the same Air x

  24. Engine oil flows through a uniformly heated pipe. The entrance length for thermal effects will be _________ the entrance length for hydrodynamic effects. longer than shorter than the same as

  25. Steam flows through a uniformly heated pipe. The entrance length for thermal effects will be _________ the entrance length for hydrodynamic effects. Consider steam to have the properties of water vapor. longer than shorter than About the same as

  26. Cold air flows between two very long parallel plates. The air enters at 25°C . The top plate is held at 70°C. The bottom plate is, initially, 25°C and insulated on one side. After some time, the bottom plate could reach _______________. 10 °C 25 °C 35 °C 70 °C 70 °C, constant Cold Air 25°C at entrance 25 °C, initially

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